Optical system

ABSTRACT

The disclosure provides an optical system, having a first optical control loop, which is set up to regulate a position and/or spatial orientation of a first optical element relative to a first module sensor frame, and a first module control loop, which is set up to regulate a position and/or spatial orientation of the first module sensor frame relative to a base sensor frame. Related components and methods are also provided

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims benefit under35 USC 120 to, international application PCT/EP2016/064086, filed Jun.17, 2016, which claims benefit under 35 USC 119 of German ApplicationNo. 10 2015 211 286.1, filed Jun. 18, 2015. The entire disclosure ofthese applications are incorporated by reference herein.

FIELD

The disclosure relates to an optical system, a photomask inspectionsystem, a projection system, a lithography apparatus and to a method forregulating an optical system.

BACKGROUND

Microlithography is used for producing microstructured components, forexample integrated circuits. The microlithography process is carried outin what is called a projection exposure apparatus, which includes anillumination device and a projection lens. The image of a photomask(reticle) illuminated by way of the illumination device is in this caseprojected by way of the projection lens onto a wafer coated with alight-sensitive layer (photoresist) and arranged in the image plane ofthe projection lens, in order to transfer the structure of the photomaskonto the light-sensitive coating of the substrate.

Mirrors of the projection lens are typically retained in a holding frame(force frame). The mirrors are positioned (in up to three translationaldegrees of freedom) and spatially oriented (in up to three rotationaldegrees of freedom) relative to a sensor frame. The sensor frame isconfigured as a stable frame surrounding the holding frame. Via openingsin the holding frame, sensor heads of the sensor frame can be broughtclosely enough to the mirrors to perform precise measurements withrespect to the position and the spatial orientation of the mirrors. Oneor more mirrors can deteriorate in terms of reflection over the courseof their operation and must be interchanged. The sensor frame can hereimpede the interchanging of deteriorated mirrors.

US 2012/0140241 A1 discloses an approach for mounting the mirrorswithout a physically stable frame. In this case, six opticallongitudinal measurement sections between in each case two neighboringmirrors are described, with which the positions and spatial orientationsof the mirrors with respect to one another can be determined. For thelongitudinal measurement sections, numerous free visual axes areinvolved, which are not always available in EUV projection lenses.

U.S. Pat. No. 7,817,248 B2 discloses an optical system, in which opticalelements are directly positioned relative to one another using referenceelements or indirectly via the holding frame to which the opticalelements are attached. The reference elements are here connected to theoptical elements.

SUMMARY

The disclosure seeks to provide an improved optical system. Inparticular, the interchanging of individual mirrors is to befacilitated.

In one aspect, the disclosure provides an optical system, having a firstoptical control loop, which is set up to regulate a position and/orspatial orientation of a first optical element relative to a firstmodule sensor frame, and a first module control loop, which is set up toregulate a position and/or spatial orientation of the first modulesensor frame relative to a base sensor frame.

The optical system preferably additionally has a second optical controlloop, which is set up to regulate a position and/or spatial orientationof a second optical element relative to a second module sensor frame,and a second module control loop, which is set up to regulate a positionand/or spatial orientation of the second module sensor frame relative tothe base sensor frame.

The position and/or spatial orientation of the first optical element canadvantageously be regulated independently of the second optical element.In other words, they do not need to have visual contact with one anotherin order to be positioned and spatially oriented in each case. Thecommon reference is here the base sensor frame.

The optical control loops and the module control loop can have differentcontrol accuracy. For example, the module control loop can providecoarse positioning, while the optical control loops serve for finepositioning. For example, for adjusting the first and second opticalelements for a lithography process, first, the coarse adjustment of thefirst and second optical elements or of the corresponding modulescontaining them can be performed using the module control loop. In afurther step, the fine adjustment of the optical elements is thenperformed using the optical control loops.

Provision may furthermore be made for the first and second opticalelements to be positioned and/or spatially oriented within therespective module thereof, and for installation of the correspondingmodules in the optical system to be performed only afterward.

It is to be understood that the first and second module sensor framesand the base sensor frame are different, i.e. spatially separate,frames.

Of course, more than two, such as six or more optical elements, plus theassociated sensor modules, can be provided.

The first and second optical elements are preferably arranged in thebeam path of the optical system, in particular in the beam path of theworking light (i.e. the light used to expose the substrate, inparticular a wafer). They can follow one another directly in the beampath, or further optical elements can be arranged in the beam pathbetween them.

The first and/or second optical element can be a mirror, a lens element,an optical grating or a waveplate.

“Frame” in the present case does not necessarily presuppose aframe-shaped structure, but rather also encompasses for example aplatform or a plate.

The first and/or second module sensor frame and/or the base sensor frameare configured to be rigid, in particular partially or completely fromone or more of the following materials: silicon carbide (SiC),reaction-bonded silicon-infiltrated silicon carbide (SiSiC), cordierite,aluminum oxide (Al₂O₃), aluminum nitride (AlN).

“Positioning” in the present context means a movement of thecorresponding optical element in up to three translational degrees offreedom. “Spatially orienting” in the present context means a movementof the corresponding optical element in up to three rotational degreesof freedom.

In accordance with one embodiment, provision is made for the firstoptical control loop to have a first sensor for capturing the positionand/or spatial orientation of the first optical element relative to thefirst module sensor frame, and a first actuator for positioning and/orspatially orienting the first optical element in dependence on thecaptured position and/or spatial orientation of the first opticalelement, and/or for the second optical control loop to have a secondsensor for capturing the position and/or spatial orientation of thesecond optical element relative to the second module sensor frame, and asecond actuator for positioning and/or spatially orienting the secondoptical element in dependence on the captured position and/or spatialorientation of the second optical element.

The first and/or second sensor preferably capture the position and/orspatial orientation of the corresponding optical element in up to sixdegrees of freedom. In particular, the position and/or spatialorientation is/are captured in contact-free fashion, for example usingoptical sensors, in particular grating sensors, or capacitive sensors.Optical capturing can be performed in the form of photoelectric scanningin accordance with the interferential measurement principle withsingle-field scanning. The resolution of preferred sensors is below 100pm, preferably below 50 pm.

The first and/or second sensor can be made up of atransmitting/receiving unit and a measurement object (target), whichreflects electromagnetic light that is emitted by thetransmitting/receiving unit back to the transmitting/receiving unit forreception. A measurement section is defined between thetransmitting/receiving unit and the measurement object. The distance canbe, for example, less than 8, preferably less than 4 and with furtherpreference less than 1 mm.

In particular, the first and/or second actuator are in the form ofLorentz, reluctance or piezoactuator or a stepper motor.

In accordance with a further embodiment, provision is made for the firstmodule control loop to have a third sensor for capturing the positionand/or spatial orientation of the first module sensor frame relative tothe base sensor frame, and a third actuator for positioning and/orspatially orienting the first module sensor frame in dependence on thecaptured position and/or spatial orientation of the first module sensorframe, and/or for the second module control loop to have a fourth sensorfor capturing the position and/or spatial orientation of the secondmodule sensor frame relative to the base sensor frame, and a fourthactuator for positioning and/or spatially orienting the second modulesensor frame in dependence on the captured position and/or spatialorientation of the second module sensor frame.

What was stated in relation to the first and second sensors and to thefirst and second actuators applies to the third and fourth sensors andthe third and fourth actuators.

In accordance with a further embodiment, provision is made for the firstmodule control loop and the first optical control loop and/or the secondmodule control loop and the second optical control loop to be set up tointeract with one another such that the position and orientation of thefirst and second optical elements are able to be regulated in each casein all six degrees of freedom relative to the base sensor frame.

In accordance with a further embodiment, the first actuator is supportedon a first module holding frame and the second actuator is supported ona second module holding frame.

The first module sensor frame can be supported on the first moduleholding frame and the second module sensor frame can be supported on thesecond module holding frame, in particular in an oscillation-decoupledmanner. They can be supported here via one or more connecting elements,which are for example soft (low spring stiffness).

In accordance with a further embodiment, the third actuator supports thefirst module holding frame on a base holding frame, and the fourthactuator supports the second module holding frame on the base holdingframe.

The base sensor frame is preferably mechanically decoupled from the baseholding frame. This means in particular that transfer of oscillationsfrom the base holding frame to the base sensor frame—for example usingsuitable dampeners—is avoided. In particular, the base sensor frame andthe base holding frame are connected to one another using an interfaceelement. The interface element can exhibit oscillation decoupling.

In particular, the base holding frame encloses a volume in which thebase sensor frame is partly or completely arranged. In this case, thebase sensor frame can also be referred to as a central sensor frame. Asa result, improved access to the optical elements, in particular for thepurposes of interchanging them, for example in the case of reflectiondeterioration, is possible. In particular, it is possible in this way tosimply install and/or interchange the modules having the first andsecond mirrors. The base sensor frame can have a plurality of armsprojecting from a base body, wherein at least two of the arms have athird sensor. The base body and the projecting arms can be configured inone part or one piece. “One part” means that the corresponding elementsare connected to form a fixedly connected unit by way of a fasteningmechanism, such as screws. “One piece” means that the correspondingelements are made of the same piece of material.

Provided in accordance with a further embodiment are a first modulehaving the first optical element, the first module sensor frame, thefirst sensor, the first module holding frame and the first actuator,and/or a second module having the second optical element, the secondmodule sensor frame, the second sensor, the second module holding frameand the second actuator, with the first and/or second module beingarranged between the base sensor frame and the base holding frame.

The first and second modules can be pre-assembled, for example, and theninstalled in the optical system, i.e. inserted between the base sensorframe and the base holding frame.

Provided in accordance with a further embodiment is a device forcapturing a change in position, a change in spatial orientation and/or adeformation of the base sensor frame or parts thereof and/or of themodule sensor frame with respect to a reference outside the base holdingframe.

In accordance with a further embodiment, the device has aninterferometer, in particular a phase-shifting interferometer or a moiréinterferometer.

In accordance with a further embodiment, the interferometer has ameasurement section, along which electromagnetic radiation is sent andwhich extends via two reflection points on the base sensor frame and/oron the module sensor frames.

In one aspect, the disclosure provides an optical system, having opticalelements, actuators, a holding frame on which the optical elements areheld in a way in which they are able to be positioned and/or spatiallyoriented using the actuators, a sensor frame which is mechanicallydecoupled from the holding frame, and sensors which are set up tocapture a position and/or spatial orientation of a respective opticalelement relative to the sensor frame, wherein the holding frame enclosesa volume and the sensor frame is arranged partially or entirely withinthe volume.

The features, definitions and further developments, previously describedin connection with the modular approach, correspondingly apply, unlessexpressly stated otherwise. The same also applies the other way around.

Due to the fact that the holding frame encloses a volume and the sensorframe is arranged partially or entirely within the volume, good accessto the optical elements is provided, for example for the purpose ofinterchanging them, without the access being blocked by the holdingframe.

The sensor frame is mechanically decoupled from the holding frame so asto avoid in particular transfer of oscillations from the holding frameto the sensor frame—for example using suitable dampeners. Alternatively,mechanical decoupling could also be dispensed with in other embodiments.

In accordance with one embodiment, provision is made for the sensorframe to have a plurality of arms projecting from a base body, whereinat least two of the arms carry in each case one of the sensors.

The resulting arm structure, which can also be referred to as a scaffoldstructure, tree structure or star structure, allows for thecorresponding measurement sections (in the present case also referred toas the “measurement distance”) for the sensors to be made small, whichincreases the measurement accuracy. What was the in relation to themodular embodiment applies correspondingly with respect to the sensors.

In accordance with a further embodiment, provision is made for the basebody and the arms projecting therefrom to be configured in one part orin one piece.

Provided in accordance with a further embodiment is a device forcapturing a change in position, a change in spatial orientation and/or adeformation of the sensor frame or parts thereof with respect to areference outside the holding frame.

In accordance with a further embodiment, the device has aninterferometer, in particular a phase-shifting interferometer or a moiréinterferometer.

In accordance with a further embodiment, the interferometer has ameasurement section, along which electromagnetic radiation is sent andwhich extends via two reflection points on the sensor frame.

Furthermore provided is a photomask inspection system for inspecting aphotomask using an optical system, as described above. Furthermore, aprojection system for a lithography apparatus including an opticalsystem, as described above, is provided.

Furthermore, a lithography apparatus including an optical system, asdescribed above, or including a projection system, as described above,is provided.

In on aspect, the disclosure provides a method for regulating an opticalsystem, wherein a position and/or spatial orientation of a first opticalelement relative to a first module sensor frame is regulated in a firstoptical control loop, and a position and/or spatial orientation of thefirst module sensor frame relative to a base sensor frame is regulatedin a first module control loop.

Furthermore provided in accordance with the method is preferably that aposition and/or spatial orientation of a second optical element relativeto a second module sensor frame is regulated in a second optical controlloop, and a position and/or spatial orientation of the second modulesensor frame relative to the base sensor frame is regulated in a secondmodule control loop.

The features and embodiments described with respect to the opticalsystem apply accordingly to the photomask inspection system, theprojection system, the lithography apparatus and the method forregulating an optical system, and the other way around.

If in the present case mention is made of “an” element, for example anoptical element or an actuator, this does not at all preclude that aplurality of corresponding elements are provided, for example 2, 3, 4 ormore.

Further aspects of an optical system, of a photomask inspection system,of a projection system, of a lithography apparatus, of a method forinstalling and/or interchanging optical elements of an optical systemwill be stated below. One or more of the aspects can be provided aloneor be combined with the optical system, the photomask inspection system,the projection system, the lithography apparatus or the method forregulating an optical system, as in each case described above.

In one aspect, the disclosure provides an optical system, which carriesa plurality of optical elements, a holding frame which carries theplurality of optical elements, a plurality of first sensors which areset up to capture a position and/or a spatial orientation of the opticalelements, and a sensor frame which carries the plurality of firstsensors. The sensor frame is arranged at least partially within theholding frame.

Due to the fact that the sensor frame is arranged at least partiallywithin the holding frame, an optical element of the optical system canbe interchanged relatively easily. This advantage is achieved because noclosed sensor frame is present anymore which surrounds the holding frameand impedes the introduction of an optical element into the opticalsystem or removal of an optical element from the optical system. Thesensor frame which is arranged at least partially within the holdingframe can furthermore be guided into the vicinity of each opticalelement, with the result that the optical elements can be positioned andspatially oriented on the basis of the sensor frame.

“Partially arranged within the holding frame” means that the holdingframe defines an enveloping surface, in particular an at least partiallycylindrical enveloping surface, into which the sensor frame projects.

The optical system has a plurality of optical elements, the positionand/or spatial orientation of which is determined by way of the sensorframe and the first sensors. The optical system can have in particulartwo, three, four, five, six, seven, eight, nine, ten, eleven or twelveof such optical elements.

The optical system can be embodied in the form of an imaging system.

A projection system of a lithography apparatus can have such an opticalsystem. The corresponding optical elements are then arranged in theprojection system of the lithography apparatus. The projection system ofthe lithography apparatus furthermore can have further optical elements,the position and/or spatial orientation of which is determined by way ofthe sensor frame and the first sensors.

In one embodiment, the first sensor has a transmitting and receivingunit and a corresponding unit that sends a signal back to thetransmitting and receiving unit. The transmitting and receiving unit ofthe first sensor is preferably attached to the sensor frame. In thiscase, the unit that sends the signal back to the transmitting andreceiving unit is arranged at the optical element or at a module havingthe optical element. Alternatively, the unit that sends the signal backto the transmitting and receiving unit can also be attached to thesensor frame. The transmitting and receiving unit of the first sensor isthen attached to the optical element or to a module having the opticalelement. At least part of the first sensor is accordingly attached tothe sensor frame.

In accordance with one embodiment of the optical system, the sensorframe has a plurality of arms. As a result, a first sensor, which isattached to one of the arms of the sensor frame, can be arranged in thevicinity of one of the optical elements. In particular, a first sensorcan be arranged at the end of an arm. This permits reliable measurementof the position and/or spatial orientation of the corresponding opticalelement. Furthermore, a sensor frame having a plurality of arms is morecompact and lightweight than a voluminous sensor frame. Due to theconstruction, it is thus possible to save weight with a sensor framehaving a plurality of arms. In addition, the sensor frame can bearranged between the optical elements. The sensor frame therefore nolonger surrounds the optical elements. Consequently, the opticalelements can be installed and removed more easily.

In accordance with a further embodiment of the optical system, thesensor frame is configured in the form of a scaffold or a star.Advantageously it is possible, due to the scaffold construction or thestar construction, to save weight as compared to a voluminous sensorframe. In addition, the sensor frame no longer surrounds the opticalelements, but is arranged between them. Consequently, the opticalelements can be installed and removed more easily.

In accordance with a further embodiment of the optical system, ameasurement distance between one of the first sensors of the sensorframe and one of the optical elements is less than 8 mm, preferably lessthan 4 mm, and with further preference less than 1 mm. The measurementdistance is the distance between the transmitting and receiving unit ofthe first sensor and the unit of the first sensor that sends a signalback to the transmitting and receiving unit. Attaching a first sensor,preferably the transmitting and receiving unit of the first sensor, tothe sensor frame in the vicinity of the optical element advantageouslypermits very accurate measurement of the position and/or of the spatialorientation of the optical element. It is possible here to use gratingsensors for the measurement. Alternatively, the use of sensors having agreater working distance would simplify the installation process.

In accordance with a further embodiment of the optical system, thesensor frame is attached to the holding frame. The holding frame isadvantageously a stable, non-deformable component. The sensor frame cantherefore be suitably attached to the holding frame.

In accordance with a further embodiment of the optical system, thelatter furthermore has an interface ring, wherein the holding frameand/or the sensor frame are attached to the interface ring. Theinterface ring is a stable, non-deformable component, to which aplurality of components can be attached. The interface ringadvantageously gives the optical system a stable construction.

In accordance with a further embodiment of the optical system, thesensor frame is configured to be rigid. “Configured to be rigid” heremeans that the sensor frame cannot be easily deformed. In this case, thesensor frame can include one or more of the following materials: siliconcarbide (SiC), reaction-bonded silicon-infiltrated silicon carbide(SiSiC), cordierite, aluminum oxide (Al₂O₃), aluminum nitride (AlN).

In accordance with a further embodiment of the optical system, thesensor frame is positionable in relation to a reference outside theholding frame. The position and/or spatial orientation of the sensorframe in relation to a reference can advantageously be readjusted. Exactpositioning and/or spatial orientation of the sensor frame is importantbecause the sensor frame for its part likewise serves as a reference forthe optical elements.

In accordance with a further embodiment of the optical system, thelatter furthermore has one or more interferometers for measuring alongitudinal change, for measuring a position change and/or formeasuring an angular change. Since the sensor frame is used as areference for positioning and/or spatially orienting the opticalelements, a deformation of the sensor frame also falsifies thepositioning and/or the spatial orientation of the optical elements. Forthis reason, the deformation of the sensor frame can be measured withone or more interferometers. The deformation of the sensor frame canthen be taken into account during the positioning and/or spatiallyorientation. In the case of a module which has an optical element, amodular sensor frame and a modular holding frame, it is furthermorepossible to measure the position and/or angular change of the modularsensor frame relative to the modular holding frame.

In accordance with a further embodiment of the optical system, the oneor more interferometers are in the form of phase-shiftinginterferometers and/or length measurement interferometers. Variousinterferometers can be advantageously used.

In accordance with a further embodiment of the optical system, the oneor more interferometers are set up to apply moiré measurementtechniques. Tilting or twisting of the sensor frame can advantageouslybe measured in this way.

In accordance with a further embodiment of the optical system, the oneor more interferometers have a branched arrangement of moiré measurementsections. It is thus advantageously possible to measure changes in theentire sensor frame, even if the sensor frame has branches.

In accordance with a further embodiment of the optical system, thelatter furthermore has a first control loop for each optical element,wherein the respective first control loop includes one or more firstactuators and one or more of the first sensors in order to position therespective optical elements relative to the sensor frame. The firstcontrol loop can be used to position and/or spatially orient the opticalelements relative to the sensor frame.

In accordance with a further embodiment of the optical system, therespective optical element is connected to the holding frame via the onefirst actuator or the plurality of first actuators. The optical elementsare advantageously carried by the holding frame.

In accordance with a further embodiment of the optical system, thelatter furthermore has at least one module, wherein the at least onemodule includes one of the optical elements, a modular sensor frameand/or a modular holding frame, and wherein the at least one module isinterchangeable. The optical element can additionally be adjustedrelative to the modular sensor frame. The modular sensor frame can beadjusted relative to the sensor frame. The optical element canfurthermore be connected to the holding frame by the modular holdingframe.

In accordance with a further embodiment of the optical system, thelatter furthermore has a second control loop for the optical element ofthe at least one module, wherein the second control loop includes one ormore second actuators and one or more second sensors in order toposition the optical element relative to the modular sensor frame. Theoptical element can advantageously be adjusted relative to the modularsensor frame using the second control loop. Overall, an optical elementis always able to be positioned and spatially oriented in six degrees offreedom, i.e. in three spatial directions and at three angles. Thisability to be positioned and spatially oriented is always the case insum with the first and second control loops. If one of the two controlloops can already position and spatially orient the optical element insix degrees of freedom, it is possible for the other control loop to beable to position and spatially orient the optical element only in fewerthan six degrees of freedom.

By way of example, the first control loop can increase the actuationrange of the mirror and thus ideally complement the second control loop,which is highly precise but in turn limited in the actuation range.

In accordance with a further embodiment of the optical system, theoptical element is connected to the modular holding frame via the onesecond actuator or the plurality of second actuators. The opticalelement is advantageously carried by the modular holding frame.

In principle, the first actuators and the second actuators can be in theform of Lorentz actuators, piezoactuators or actuators having steppermotors. In accordance with a further embodiment of the optical system,the modular holding frame is connected to the holding frame.Accordingly, the optical element is connected directly to the modularholding frame via the second actuators. The optical element isfurthermore indirectly connected to the holding frame via the modularholding frame and the first actuators, which are attached both to themodular holding frame and to the holding frame.

In accordance with a further embodiment of the optical system, one ormore of the first and/or one or a plurality of the second sensors areconfigured in the form of optical sensors. Optical sensors areadvantageously highly suitable for vacuum environments.

In accordance with a further embodiment of the optical system, thesensor frame defines a coordinate system and the respective opticalelement is positionable in three spatial directions and three anglesrelative to the coordinate system using the first and/or second controlloop. “Defining” means that the sensor frame acts as a reference pointfor the coordinate system. The optical element is always capable ofbeing positioned and spatially oriented in six degrees of freedom. Thisability to be positioned and spatially oriented can be achieved by wayof the first and/or second control loop. The ability to be positionedand spatially oriented in six degrees of freedom, i.e. the ability to bepositioned and spatially oriented in three spatial directions and atthree angles, is always achieved in the sum of the first and secondcontrol loops.

In accordance with a further embodiment of the optical system, therespective optical element has a mirror or a lens element. The opticalelement can be configured both in the form of a mirror and in the formof a lens element.

Furthermore described is a method for installing and/or interchangingoptical elements of an optical system. The method here has the followingsteps: a) inserting at least one optical element into the optical systemby connecting it to a holding frame, b) measuring a position and/orspatial orientation of the at least one optical element relative to asensor frame, wherein the sensor frame is arranged at least partiallywithin the holding frame, c) positioning and/or spatially orienting theat least one optical element relative to the sensor frame in accordancewith the measurement result from step b), and d) fixing the positionedand/or spatially oriented at least one optical element.

Due to the fact that the sensor frame is arranged at least partiallywithin the holding frame, an optical element of the optical system canbe installed or interchanged relatively easily.

In accordance with an embodiment of the method, the following step isperformed before step a): removing at least one optical element from theoptical system.

In accordance with an embodiment of the method, the measurement of theposition and/or spatial orientation of the at least one optical elementin step b) is performed in a contact-free manner. Due to thecontact-free measurement, no forces are transferred to the opticalelement during the measurement.

In accordance with a further embodiment of the method, the measurementof the position and/or spatial orientation of the at least one opticalelement in step b) is performed using one or more optical sensors.Optical sensors are advantageously highly suitable for vacuumenvironments.

In accordance with a further embodiment of the method, a module isinserted in step a) into the optical system, which module includes theat least one optical element, a modular sensor frame and/or a modularholding frame, wherein the module is interchangeable. Advantageously,the entire module can be installed in and removed from the opticalsystem.

The embodiments and features described for the proposed optical systemare correspondingly applicable to the proposed method. Furthermoreproposed is a photomask inspection system for inspecting a photomaskusing an optical system, as described. The photomask can be inspectedfor errors using the photomask inspection system.

Furthermore proposed is a projection system for a lithography apparatusincluding an optical system, as described.

Furthermore, a lithography apparatus including a projection system, asdescribed, or an optical system, as described, is proposed.

Further possible implementations of the disclosure also include notexplicitly mentioned combinations of features or embodiments that aredescribed above or below with respect to the exemplary embodiments. Inthis respect, a person skilled in the art will also add individualaspects to the respective basic form of the disclosure as improvementsor additions.

Further advantageous configurations and aspects of the disclosure arethe subject of the dependent claims and also of the exemplaryembodiments of the disclosure described below.

BRIEF DESCRIPTION OF THE DRAWINGS

In the text that follows, the disclosure is explained in more detail onthe basis of preferred embodiments with reference to the accompanyingfigures, in which:

FIG. 1 shows a schematic view of an EUV lithography apparatus;

FIG. 2 shows a schematic view of an optical system in accordance with afirst exemplary embodiment;

FIG. 3 shows a schematic view of an optical system in accordance with asecond exemplary embodiment;

FIG. 3a shows a section from FIG. 3;

FIG. 4 shows a schematic view of an optical system in accordance with athird exemplary embodiment;

FIG. 5 shows a schematic view of a part of an optical system inaccordance with a fourth exemplary embodiment;

FIG. 6 shows a schematic view of a part of an optical system inaccordance with a fifth exemplary embodiment;

FIG. 7 shows a schematic view of a part of an optical system inaccordance with a sixth exemplary embodiment;

FIG. 8 shows a schematic view of a part of an optical system inaccordance with a seventh exemplary embodiment;

FIG. 9 shows a schematic view of a part of an optical system inaccordance with an eighth exemplary embodiment;

FIG. 10 shows a schematic view of a part of an optical system inaccordance with a ninth exemplary embodiment; and

FIG. 11 shows a flowchart of a method for installing and/orinterchanging mirrors of an optical system.

DETAILED DESCRIPTION

Unless otherwise indicated, the same reference signs in the figuresdenote elements that are the same or functionally the same. It shouldalso be noted that the illustrations in the figures are not necessarilyto scale.

FIG. 1 shows a schematic view of an EUV lithography apparatus 100according to one embodiment, which includes a beam shaping system 102,an illumination system 104 and a projection system 106. The beam shapingsystem 102, the illumination system 104 and the projection system 106are respectively provided in a vacuum housing, which is evacuated withthe aid of an evacuation device that is not depicted in any more detail.The vacuum housings are surrounded by a machine room (not depicted inany more detail), in which the drive apparatuses for mechanically movingor adjusting the optical elements are provided. Moreover, electricalcontrollers and the like can also be provided in this machine room.

The beam shaping system 102 has an EUV light source 108, a collimator110 and a monochromator 112. A plasma source or a synchrotron, whichemit radiation in the EUV range (extreme ultraviolet range), that is tosay for example in the wavelength range from 0.1 nm to 30 nm, may forexample be provided as the EUV light source 108. The radiation emittedby the EUV light source 108 is first focused by the collimator 110,after which the desired operating wavelength is filtered out by themonochromator 112. Consequently, the beam shaping system 102 adapts thewavelength and the spatial distribution of the light emitted by the EUVlight source 108. The EUV radiation 114 generated by the EUV lightsource 108 has a relatively low transmissivity through air, for whichreason the beam guiding spaces in the beam shaping system 102, in theillumination system 104 and in the projection system 106 are evacuated.

In the depicted example, the illumination system 104 includes a firstmirror 116 and a second mirror 118. These mirrors 116, 118 may forexample be formed as facet mirrors for pupil shaping and conduct the EUVradiation 114 to a photomask 120.

The photomask 120 is likewise formed as a reflective optical element andmay be arranged outside the systems 102, 104, 106. The photomask 120 hasa structure which is imaged onto a wafer 122 or the like in a reducedfashion via the projection system 106. For this purpose, the projectionsystem 106 has in the beam guiding space for example a third mirror 124and a fourth mirror 126. It should be noted that the number of mirrorsof the EUV lithography apparatus 100 is not restricted to the numberrepresented. A greater or lesser number of mirrors can also be provided.Furthermore, the mirrors, as a rule, are curved on their front side forbeam shaping.

FIG. 2 shows a schematic view of an optical system 200 in accordancewith a first exemplary embodiment. The optical system 200 is, forexample, part of the EUV lithography apparatus 100 illustrated in FIG. 1or, more specifically, part of the projection system 106 illustrated inFIG. 1. Alternatively, the optical system 200 can also be part of theillumination system 104 illustrated in FIG. 1.

The optical system 200 has a holding frame 202 (in the present case also“base holding frame”), a sensor frame 204 (in the present case also“base sensor frame”) and, by way of example, optical elements in theform of the two mirrors 124, 126. The optical system 200 furthermore hastwo sensors 206 a, 206 b. A corresponding modular sensor frame 208 a,208 b and a corresponding modular holding frame 210 a, 210 b areprovided for each of the two mirrors 124, 126.

A sensor 206 a, 206 b has a transmitting and receiving unit 212 and acorresponding measurement object 214, which sends an optical signal backto the transmitting and receiving unit 212. The position and/or spatialorientation of one of the mirrors 124, 126 can be determined on thebasis of the signal that is sent back. The transmitting and receivingunit 212 of the sensor 206 a, 206 b is preferably attached to the sensorframe 204. In this case, the measurement object 214, which sends thesignal back to the transmitting and receiving unit 212, is arranged atthe respective modular sensor frame 208 a, 208 b, which is associatedwith the corresponding mirror 124, 126. Alternatively, the measurementobject 214 can also be attached to the sensor frame 204. Thetransmitting and receiving unit 212 is then attached to the modularsensor frame 208 a, 208 b, which is associated with the correspondingmirror 124, 126. At least part of a respective sensor 206 a, 206 b isaccordingly attached to the sensor frame 204.

The optical system 200 has a module control loop 216 a, 216 b for eachof the two mirrors 124, 126. Each of the module control loops 216 a, 216b depicted in FIG. 2 includes actuators 218 a, 218 b and the sensors 206a, 206 b. The module control loops 216 a, 216 b can be used to positionand/or spatially orient the respective mirror 124, 126 together with themodular sensor frame 208 a, 208 b and the modular holding frame 210 a,210 b relative to the sensor frame 204. The modular holding frames 210a, 210 b are here connected to the holding frame 202 via the actuators218 a, 218 b.

The optical system 200 depicted in FIG. 2 furthermore has an opticalcontrol loop 233 a, 233 b for each of the two mirrors 124, 126. Each ofthe two optical control loops 233 a, 233 b illustrated includesactuators 222 a, 222 b and sensors 224 a, 224 b. The optical controlloops 233 a, 233 b can be used to position and/or spatially orient therespective mirror 124, 126 relative to the modular sensor frame 208 a,208 b. The mirrors 124, 126 are here connected to the modular holdingframe 210 a, 210 b via the second actuators 222 a, 222 b.

The sensor 224 a, 224 b has a transmitting and receiving unit 226 and acorresponding measurement object 228, which sends a signal back to thetransmitting and receiving unit 226. The position and/or spatialorientation of one of the mirrors 124, 126 relative to the modularsensor frame 208 a, 208 b can be determined on the basis of the signalthat is sent back. The transmitting and receiving unit 226 is preferablyattached to the modular sensor frame 208 a, 208 b. In this case, themeasurement object 228 is arranged at the mirror 124, 126.Alternatively, the arrangement can also be the other way around. Atleast part of the sensor 224 a, 224 b is accordingly attached to themodular sensor frame 208 a, 208 b.

One of the mirrors 124, 126, the corresponding modular sensor frame 208a, 208 b and/or the corresponding modular holding frame 210 a, 210 b canin each case form a module 232 a, 232 b. The respective module 232 a,232 b can be installed and removed from the optical system 200 as acomponent.

Alternatively, the optical system 200 does not have a modular holdingframe 210 a, 210 b and/or a modular sensor frame 208 a, 208 b for eachmirror 124, 126 (or for no mirror 124, 126). Positioning and/or spatialorientation of the mirrors 124, 126 is performed for the mirrors 124,126 without associated modular holding frame 210 a, 210 b and modularsensor frame 208 a, 208 b only via the optical control loops 233 a, 233b.

The mirrors 124, 126 are always capable of being positioned andspatially oriented in six degrees of freedom. This ability to bepositioned and spatially oriented can be achieved by way of the moduleand/or optical control loop 216 a, 216 b, 233 a, 233 b. In the sum, theability to be positioned and spatially oriented in six degrees offreedom, i.e. the ability to be positioned and spatially oriented inthree spatial directions and at three angles, is always achieved by wayof the control loops 216 a, 216 b, 233 a, 233 b.

The actuators 218 a, 218 b and the actuators 222 a, 222 b form acascaded system. By way of example, the module control loop 216 a, 216 bcan increase the actuation range of the mirror 124, 126 and thus ideallycomplement the optical control loop 233 a, 233 b, which is highlyprecise but in turn limited in the actuation range. This permits bothcoarse and fine adjustment.

The sensor frame 204 is arranged partially or completely within a volumeV (see FIGS. 3 and 3 a, wherein the latter depicts a section IIIa-IIIafrom FIG. 3) which is enclosed by the holding frame 202. By way ofexample, the holding frame 202 can enclose an at least partiallycylindrical, in particular circular-cylindrical volume V, as can be seenfrom figures 3 and 3 a together. As a result, a mirror 124, 126 of theoptical system 200 can be easily installed or interchanged. Thisadvantage is achieved because no closed sensor frame is present anymorewhich would surround the holding frame 202 and impede the introductionof a mirror 124, 126 into the optical system 200 or removal of a mirror124, 126 from the optical system 200. The sensor frame 204 canfurthermore be arranged between the mirrors 124, 126. It is possible inthis way for the sensor frame 204 to be guided into the vicinity of eachmirror 124, 126 or of each modular sensor frame 208, which is associatedwith a mirror 124, 126. As a result, the mirrors 124, 126 can bepositioned and spatially oriented on the basis of the sensor frame 204.

The respective modular sensor frame 208 a, 208 b can be connected to therespective modular holding frame 210 a, 210 b via in particularoscillation-decoupling connecting elements 230 (see FIG. 2). The sensors206 a, 206 b, 224 a, 224 b can be in the form of optical sensors. Theoptical system 200 can also have lens elements or other optical elementsinstead of or in addition to the mirrors 124, 126.

FIG. 3 shows a schematic view of an optical system 200 in accordancewith a second exemplary embodiment. In this exemplary embodiment, thesensor frame 204 is arranged entirely within the holding frame 202. TheEUV radiation in the holding frame 202 passes via holes 300 to themirrors 124, 126 and out of the holding frame 202. As opposed to theexemplary embodiment from FIG. 2, the exemplary embodiment of FIG. 3 hasno modular holding frames 210 a, 210 b for the mirrors 124, 126. FIG. 3does not depict cascaded actuators either. It does show the actuators222 a, 222 b for the optical control loop 233 a, 233 b. It wouldalternatively also be possible to provide the actuators 218 a, 218 b forthe module control loop 216 a, 216 b. The mirror 124, 126 can thus bepositioned and/or spatially oriented relative to the sensor frame 204and/or relative to the respective modular sensor frame 208 a, 208 b.

The sensor frame 204 has a base body 301 and, projecting therefrom, afirst arm 302, a second arm 304, and a third arm 306. Consequently, thesensors 206 a, 206 b can be arranged near the modular sensor frames 208a, 208 b. Alternatively, the sensor frame can also be configured in theform of a scaffold. In a further alternative, a plurality of arms of thesensor frame 204 can form a star shape. The arms 302, 304, 306 and thebase body 301 are configured in one part or in one piece.

The measurement distance, or the measurement section, 308 is thedistance between the transmitting and receiving unit 212 and themeasurement body 214 and is less than 8 mm, preferably less than 4 mmand with further preference less than 1 mm. A small measurement distance308 permits very accurate measurement of the position and/or of thespatial orientation of the modular sensor frame 208 and thus of themirror 124, 126. The small measurement distance 308 is achieved by wayof the arms 302, 304, 306 having the sensors 206 a, 206 b and reachingto the modular sensor frames 208 a, 208 b.

The sensor frame 204 shown in FIG. 3 is attached to the holding frame202, possibly via a mechanical insulation (flexible connection) (notillustrated). Attachment can be effected by way of an interface ring(not shown).

FIG. 4 shows a schematic view of an optical system 200 in accordancewith a third exemplary embodiment. The third exemplary embodimentdiffers from the second exemplary embodiment in that no modular sensorframe 208 a, 208 b is associated with the mirrors 124, 126. An opticalcontrol loop 233 a, 233 b can be used to position and/or spatiallyorient a mirror 124, 126 relative to the sensor frame 204. Here, theoptical control loop 233 a, 233 b has actuators 222 a, 222 b and sensors206 a, 206 b.

FIG. 5 shows a schematic view of a part 500 of an optical system 200 inaccordance with a fourth exemplary embodiment. Mirrors 124, 126 are notshown. Illustrated are the holding frame 202 and the sensor frame 204.The sensor frame 204 is measured using a phase-shifting interferometer502. The phase-shifting interferometer 502 has an interferometercomponent 504, which is positioned in a defined manner with respect to areference 501 outside the holding frame 202, a measurement mirror 506and an optical component 508. The interferometer component 504 isarranged outside the holding frame 202. Electromagnetic radiation,illustrated by way of a first ray 512 and a second ray 514, is directedthrough an opening 510 via a deflection mirror 516 onto the measurementmirror 506. In the process, the first ray 512 and the second ray 514pass through the optical component 508.

The measurement mirror 506 and the optical component 508 are fixedlyconnected to the sensor frame 204. The optical component 508 has, on itsside facing the measurement mirror 506, a reference surface 518. Thereference surface 518 is inclined relative to the measurement mirror506. The radiation reflected at the measurement mirror 506, illustratedby way of a third ray 520 and a fourth ray 522, is directed via thedeflection mirror 516 and through the opening 510 back into theinterferometer component 504. In the process, the radiation passes for asecond time through the optical component 508. Owing to the referencesurface 518 being inclined relative to the measurement mirror 506, thethird and fourth rays 520, 522 have different optical paths and phases.As a consequence, an interferogram 524 can be seen in the interferometercomponent 504. The different optical paths and phases of the third andfourth rays 520, 522 are symbolized by way of the returning fourth ray522 starting only at the reference surface 518.

A measurement section 526 is located between the measurement mirror 506and the optical component 508. If the length of the sensor frame 204changes, the length of the measurement section 526 will also change.This change in length can be read in the interferogram 524.

FIG. 6 shows a schematic view of a part 500 of an optical system 200 inaccordance with a fifth exemplary embodiment. As opposed to the fourthexemplary embodiment shown in FIG. 5, the fifth exemplary embodimentshows the modular sensor frame 208 a and the modular holding frame 210a. If the position of the modular sensor frame 208 a and/or the positionof the modular holding frame 210 a changes, the length of themeasurement section 526 will also change. This change in length can beread in the interferogram 524.

FIG. 7 shows a schematic view of a part 500 of an optical system 200 inaccordance with a sixth exemplary embodiment. Mirrors 124, 126 are notshown. Illustrated are the holding frame 202 and the sensor frame 204.The sensor frame 204 is measured using an interferometer 600 with moirémeasurement technology. The interferometer 600 with moiré measurementtechnology has a camera 602, a concave mirror 604 and a grating 606. Thecamera 602 is arranged outside the holding frame 202. A light source 608is likewise arranged outside the holding frame 202. Electromagneticradiation from the light source 608 is directed through an opening 510,via a deflection mirror 516, onto the left-hand part 610 of the grating606. The left-hand part 610 of the grating 606 is imaged, by way of theconcave mirror 604, onto the right-hand part 612 of the grating 606.This gives a moiré pattern, which is recorded, via the deflection mirror516 and an observation optical unit 614, by the camera 602.

The concave mirror 604 is fixedly connected to the sensor frame 204using a connecting element 616. The grating 606 is likewise fixedlyconnected to the sensor frame 204. If the sensor frame 204 bends, theconcave mirror 604 will be tilted. This is symbolized by way of thecurved double-headed arrow 618. As a result, the moiré measurementsection 620 is lengthened or shortened, and the image of the left-handpart 610 of the grating 606 will be shifted to the right-hand part 612of the grating 606. This effects a change in the moiré pattern, which isdetected by way of the camera 602.

FIG. 8 shows a schematic view of a part 500 of an optical system 200 inaccordance with a seventh exemplary embodiment. As opposed to the sixthexemplary embodiment shown in FIG. 7, the seventh exemplary embodimentshows the modular sensor frame 208 a and the modular holding frame 210a. If the position and/or the spatial orientation of the modular sensorframe 208 a and/or the position and/or the spatial orientation of themodular holding frame 210 a changes, then the length of the moirémeasurement section 620 will also change and the image of the left-handpart 610 of the grating 606 will be shifted onto the right-hand part 612of the grating 606. This effects a change in the moiré pattern, which isdetected by way of the camera 602.

FIG. 9 shows a schematic view of a part 500 of an optical system 200 inaccordance with an eighth exemplary embodiment. The eighth exemplaryembodiment differs from the sixth exemplary embodiment in FIG. 7 in thata plane mirror 700 is provided on the sensor frame 204 in the eighthexemplary embodiment. In the eighth exemplary embodiment, once again aleft-hand part 610 of a grating 606 is imaged onto a right-hand part 612of the grating 606, and the resulting moiré pattern is detected.However, the radiation is deflected via the plane mirror 700. Hereby, atwisting measurement section is produced using the moiré measurementtechnique. The main idea is here that the obliquely illuminated planemirror 700 rotates the image of the left-hand part 610 of the grating606 if it is tilted about an axis located in the plane of incidence.

FIG. 10 shows a schematic view of a part 500 of an optical system 200 inaccordance with a ninth exemplary embodiment. As opposed to the eighthexemplary embodiment shown in FIG. 9, the ninth exemplary embodimentshows the modular sensor frame 208 a and the modular holding frame 210a. The plane mirror 700 is here arranged on the modular sensor frame 208a. If the position and/or the spatial orientation of the modular sensorframe 208 a and/or the position and/or the spatial orientation of themodular holding frame 210 a changes, the moiré pattern will also change.

FIG. 11 shows a flowchart of a method for installing and/orinterchanging mirrors 124, 126 of an optical system 200. In a first stepS1, one of the mirrors 124, 126 is inserted into the optical system 200.In a second step S2, the position and/or spatial orientation of themirror 124, 126 relative to a sensor frame 204 is measured. The sensorframe 204 is here arranged at least partially within a holding frame202. In a third step S3, the mirror 124, 126 is positioned and/orspatially oriented relative to the sensor frame 204 in accordance withthe measurement result according to step 2. In a fourth step S4, thepositioned and/or spatially oriented mirror 124, 126 is secured.

The measurement of the position and/or spatial orientation of the mirror124, 126 in step S2 can be effected in a contact-free manner.Furthermore, the measurement of the position and/or spatial orientationof the mirror 124, 126 in step S2 can be effected using one or moreoptical sensors 206 a, 206 b, 224 a, 224 b.

Exemplary embodiments for an optical system 200 of an EUV lithographyapparatus with a wavelength of the operating light of between 0.1 and 30nm have been explained. However, the disclosure is not restricted to EUVlithography apparatuses and may also be applied to other lithographyapparatuses. A DUV (deep ultraviolet) lithography apparatus having awavelength of the operating light of between 30 and 250 nm is mentionedhere by way of example. The optical system 200 can furthermore also beused in a photomask inspection system for inspecting a photomask 120.

Although the disclosure has been described on the basis of variousexemplary embodiments, it is not in any way restricted to them butrather can be modified in a wide variety of ways.

LIST OF REFERENCE SIGNS

100 EUV lithography apparatus

102 Beam shaping system

104 Illumination system

106 Projection system

108 EUV light source

110 Collimator

112 Monochromator

114 EUV radiation

116 First mirror

118 Second mirror

120 Photomask

122 Wafer

124 Third mirror

126 Fourth mirror

200 Optical system

202 Holding frame

204 Sensor frame

206 a, 206 b Sensor

208 a, 208 b Modular sensor frame

210 a, 210 b Modular holding frame

212 Transmitting and receiving unit

214 Measurement object

216 a, 216 b Module control loop

218 a, 218 b Actuator

222 a, 222 b Actuator

224 a, 224 b Sensor

226 Transmitting and receiving unit

228 Measurement object

230 Connecting element

232 a, 232 b Module

233 a, 233 b Optical control loop

300 Hole

301 Base body

302 First arm

304 Second arm

306 Third arm

308 Measurement distance

500 Part

501 Reference

502 Phase-shifting interferometer

504 Interferometer component

506 Measurement mirror

508 Optical component

510 Opening

512 First ray

514 Second ray

516 Deflection mirror

518 Reference surface

520 Third ray

522 Fourth ray

524 Interferogram

526 Measurement section

600 Interferometer with moiré measurement technology

602 Camera

604 Concave mirror

606 Grating

608 Light source

610 Part of the grating

612 Part of the grating

614 Observation optical unit

616 Connecting element

618 Curved double-headed arrow

620 Moiré measurement section

700 Plane mirror

V Volume

1. An optical system, wherein at least one of the following holds: i)the optical system comprises: a first optical control loop configured toregulate a position and/or spatial orientation of a first opticalelement relative to a first module sensor frame; and a first modulecontrol loop configured to regulate a position and/or spatialorientation of the first module sensor frame relative to a base sensorframe; and ii) the optical system comprises: a second optical controlloop configured to regulate a position and/or spatial orientation of asecond optical element relative to a second module sensor frame; and asecond module control loop configured to regulate a position and/orspatial orientation of the second module sensor frame relative to thebase sensor frame.
 2. The optical system of claim 1, wherein at leastone of the following holds: the first optical control loop comprises: afirst sensor configured to capture the position and/or spatialorientation of the first optical element relative to the first modulesensor frame; and a first actuator configured to position and/orspatially orient the first optical element in dependence on the capturedposition and/or spatial orientation of the first optical element; andthe second optical control loop comprises: a second sensor configured tocapture the position and/or spatial orientation of the second opticalelement relative to the second module sensor frame; and a secondactuator configured to position and/or spatially orient the secondoptical element in dependence on the captured position and/or spatialorientation of the second optical element.
 3. The optical system ofclaim 2, wherein at least one of the following holds: the first modulecontrol loop comprises: a third sensor configured to capture theposition and/or spatial orientation of the first module sensor framerelative to the base sensor frame; and a third actuator configured toposition and/or spatially orient the first module sensor frame independence on the captured position and/or spatial orientation of thefirst module sensor frame; and the second module control loop comprises:a fourth sensor configured to capture the position and/or spatialorientation of the second module sensor frame relative to the basesensor frame; and a fourth actuator configured to position and/orspatially orient the second module sensor frame in dependence on thecaptured position and/or spatial orientation of the second module sensorframe.
 4. The optical system of claim 3, wherein at least one of thefollowing holds: the first module control loop and the first opticalcontrol loop are configured to interact with each other so that theposition and orientation of the first and second optical elements areregulatable in each case in all six degrees of freedom relative to thebase sensor frame; and the second module control loop and the secondoptical control loop are configured to interact with each other so thatthe position and orientation of the first and second optical elementsare regulatable in each case in all six degrees of freedom relative tothe base sensor frame.
 5. The optical system of claim 4, wherein atleast one of the following holds: the third actuator supports the firstmodule holding frame on a base holding frame, and the fourth actuatorsupports the second module holding frame on the base holding frame; thefirst module comprises the first optical element, the first modulesensor frame, the first sensor, the first module holding frame and thefirst actuator; the second module comprises the second optical element,the second module sensor frame, the second sensor, the second moduleholding frame and the second actuator; and the first and/or secondmodules is/are interchangeable between the base sensor frame and thebase holding frame.
 6. The optical system of claim 1, wherein both i)and ii) hold. 7.-10. (canceled)
 11. An inspection system, comprising: anoptical system according to claim 1, wherein the inspection system isconfigured to inspect a photomask.
 12. A projection system, comprising:an optical system according to claim 1, wherein the projection system isa lithography projection system.
 13. An apparatus, comprising: anoptical system according to claim 1, wherein the apparatus is alithography apparatus. 14.-22. (canceled)
 23. A method for regulating anoptical system, the method comprising at least one of the following: i)regulating a position and/or spatial orientation of a first opticalelement relative to a first module sensor frame in a first opticalcontrol loop, and regulating a position and/or spatial orientation ofthe first module sensor frame relative to a base sensor frame in a firstmodule control loop; and ii) regulating a position and/or spatialorientation of a second optical element relative to a second modulesensor frame in a second optical control loop, and regulating a positionand/or spatial orientation of the second module sensor frame relative tothe base sensor frame in a second module control loop.
 24. The opticalsystem of claim 5, wherein both i) and ii) hold.
 25. The optical systemof claim 4, wherein both i) and ii) hold.
 26. The optical system ofclaim 3, wherein both i) and ii) hold.
 27. The optical system of claim3, wherein at least one of the following holds: the third actuatorsupports the first module holding frame on a base holding frame, and thefourth actuator supports the second module holding frame on the baseholding frame; the first module comprises the first optical element, thefirst module sensor frame, the first sensor, the first module holdingframe and the first actuator; the second module comprises the secondoptical element, the second module sensor frame, the second sensor, thesecond module holding frame and the second actuator; and the firstand/or second modules is/are interchangeable between the base sensorframe and the base holding frame.
 28. The optical system of claim 2,wherein both i) and ii) hold.
 29. The optical system of claim 2, whereinat least one of the following holds: the first module control loop andthe first optical control loop are configured to interact with eachother so that the position and orientation of the first and secondoptical elements are regulatable in each case in all six degrees offreedom relative to the base sensor frame; and the second module controlloop and the second optical control loop are configured to interact witheach other so that the position and orientation of the first and secondoptical elements are regulatable in each case in all six degrees offreedom relative to the base sensor frame.
 30. The optical system ofclaim 1, wherein i) holds.
 31. The optical system of claim 1, whereinii) holds.
 32. The optical system of claim 1, wherein at least one ofthe following holds: the first module control loop and the first opticalcontrol loop are configured to interact with each other so that theposition and orientation of the first and second optical elements areregulatable in each case in all six degrees of freedom relative to thebase sensor frame; and the second module control loop and the secondoptical control loop are configured to interact with each other so thatthe position and orientation of the first and second optical elementsare regulatable in each case in all six degrees of freedom relative tothe base sensor frame.
 33. The optical system of claim 1, wherein atleast one of the following holds: i) the first module control loopfurther comprises a first actuator configured to position and/orspatially orient the first optical element, wherein the first actuatoris supported on a first module holding frame; and ii) the second opticalcontrol loop further comprises a second actuator configured to positionand/or spatially orient the second optical element, wherein the secondactuator is supported on a second module holding frame.